Most integrated circuits need a timing device, or timer, to initiate certain operations regularly. For example, an integrated circuit of a dynamic random access memory (DRAM) equipped with a self refresh needs a timer to initiate the refresh operation. This timer measures the time interval between refresh operations, or elapsed time, to regularly trigger the refresh operation. There are many different designs of timers, but generally a timer for a refresh operation of a DRAM is usually constructed using a ring oscillator. The ring oscillator is usually fabricated on the same integrated circuit within the DRAM.
A typical ring oscillator mainly comprises an odd number of inverter stages connecting serially in a ring fashion with the output of each inverter connected to the input of the succeeding inverter in the ring. The output of the last inverter is connected to the input of the first inverter to produce an oscillating signal or oscillation frequency.
Those familiar with designs fabricated on integrated circuits know that the speed at which these designs operate is influenced by changes in supply voltage and operating temperature. Specifically, these designs operate at a higher speed with increasing supply voltage and/or decreased temperature and at a lower speed at the opposite extremes. This is largely due the changes in transistor conductance over voltage and temperature.
The oscillation frequency of the ring oscillator is influenced by the changes in voltage and temperature. When the temperature increases and/or the supply voltage decreases, the effective carrier mobility in the channel of the transistors decreases, the transistors become less conductive, and the speed of the inverter stages is reduced, which consequently contributes to the decrease of the oscillation frequency. At the other extreme, when the temperature decreases and/or supply voltage increases, the transistors are more conductive and the speed of the inverter stages is faster; therefore the oscillation frequency is increased.
For the reasons stated above, there is a need for an improved ring oscillator with a frequency stabilizing circuit so that the frequency of the ring oscillator is stabilized when there is a change is voltage and temperature.